There presently exist several options for use in concentrating solutions used in biomedical research and development. Such solutions are aqueous in many instances, but may also frequently contain organic solvents that are miscible with water. Evaporation of organic solvents having low boiling points can often be performed rising a standard "rotavapor" type device.
Methods commonly used to remove water from solutions containing biopolymers often involve lyophilization or freeze drying. In such a method the sample is frozen in a tube or a flask, and a vacuum is applied. The removal of water is then performed from a solid state (e.g., ice) into a receiving flask, which is also cooled to collect the water vapor.
When an aqueous solution also contains an organic solvent that is miscible with water, it may not be possible to perform a lyophilization procedure, particularly if the mixture does not solidify upon cooling. In this case a number of other evaporation techniques can be used.
Such a solution can be subjected to a vacuum under conditions that prevent the sample from "bumping" (i.e., boiling in a manner that causes the solution to splash rapidly). The solution can then be agitated or rotated to generate a centrifugal force. Instruments suitable for such purposes can provide a combination of gyrating motion, heat, and either vacuum or blow-down. Certain instruments suitable for such purposes are generally referred to as centrifugal concentrators and are among the most common evaporators in laboratories.
With such evaporators, the evaporation process is generally facilitated by the use of a heating source, in fact, the vacuum chamber generally includes a thermostat-controlled heating device. The vacuum needed to use such evaporators is typically provided by the use of a high-vacuum pump that is capable of generating pressure down to at least 1 mm Hg.
A simple evaporation process can be achieved by the use of a heating block such as the "Reacti-Therm" dry block available from Pierce as product #18800/18801. In such a process a tube containing a sample is inserted into a block that can be heated to a desired temperature. A more efficient evaporation is achieved in the heated block if a gas is streaming through to carry the gaseous solvent with it. These types of evaporators are commonly called blow-down evaporators.
The evaporation units presently available, however, often encounter significant drawbacks. Generally, the evaporation units that are the least expensive and easiest to use are the same units that require the longest evaporation times, or have other associated drawbacks. In contrast, the more expensive and technically complicated units are often unnecessary for simple procedures and cost-prohibitive for many labs.
In response to these problems, Applicants have previously provided a simple and inexpensive, but highly effective, evaporation apparatus described in co-pending application Ser. No. 08/209,786. The apparatus is useful for evaporating solvents from a plurality of solvent-containing sample vials, and includes the use of a heat-transmissive block having a number of well positions that are used to hold sample vials within a chamber in the block.
The chamber can be covered by a cover to form a substantially air tight seal. The cover, in turn, has a corresponding number of access apertures for delivering air to the underlying vial. The block is also fitted with a vacuum system that can produce a vacuum within the chamber. The vacuum created within the chamber is used to pull air through the apertures and into the underlying vial, resulting in a combined vacuum/blow-down evaporative effect. Optionally, the block itself can be heated in order to further promote the process.
Generally each of the evaporation methods and apparatuses described above involve the use of tubes that are open to the ambient environment. Measures must often be employed to avoid or minimize cross-contamination between samples that may occur through bumping or splashing of the samples.
For instance, problems may arise when ammonium hydroxide is evaporated without pre-cooling, due to the high vapor pressure of ammonia. In such cases a strong vacuum can rapidly degas the solution in such a manner as to splash the contents from the vial and into the vacuum chamber. Bumping can also occur when the sample is cooled, since it is difficult to control the gaseous mist generated during the evacuation of the vacuum chamber used for the evaporation.
The evaporative effect created by a centrifugal vacuum evaporator is generally slow, as compared with a vacuum-blow down unit that furnishes a steady steam of air or inert gas through the vial or tube. This slower evacuation and concentration of the sample allows a gas-phase mist to equilibrate with the solution in the vial. As a result, however, biomolecules may be suspended in the mist, which potentially can enter and contaminate other tubes.
Many, if not all, biochemical procedures are quite sensitive to the problem of cross-contamination from adjacent or nearby sample tubes. For instance, ever-increasing sensitivity is provided by gene amplification techniques, such as the PCR technique described by U.S. Pat. Nos. 4,683,195 and 4,683,202 (Hoffmann La-Roche Inc.). Such techniques, however, require that the DNA sequences used in the reaction be completely free of other sequences. Otherwise, the PCR technique would tend to amplify any such contaminant in the final product.
In another context, quite distinct from evaporation, a number of products have been introduced over recent years for the prevention of aerosol contamination in the use of pipettes and pipette tips. Such products include "XCLUDA" aerosol barrier pipet tips from BioRad, FILTERTIPS (hydrophobic polyethylene) from Eppendorf, ABSOLIFE filters (melt blown polypropylene) from Gelman Sciences, AERO-GARD brand aerosol barrier tips, AEROSHIELD filter piper tips from Robbins Scientific, and AEROSEAL GOLD brand filter tips available from USA Scientific Plastics, Inc.
One such product for use in pipets is known as "ART" (aerosol resistant tips), available from Molecular Bio-Products (San Diego). Such tips are described in U.S. Pat. No. 5,156,811, the disclosure of which is incorporated herein by reference. This product is described as having a porous plug, formed of hydrophobic polyethylene, that is impregnated with particles of a liquid scavenging material. If any pores of the plug contact vaporized liquid droplets, the scavenging material swells so as to block gas or liquid flow through those particular pores.
What is clearly needed is an evaporation unit and method that provides an optimal combination of cost, ease of use, and efficiency for evaporating aqueous solvents commonly encountered in biomedical research.